Patterning of radiation sensitive polymeric films (referred to as photoresists) using photons, electrons, or ion beams is a critical step in the manufacture of semiconductor devices. The incident radiation includes commonly used wavelengths of 436, 365, 257, 248, 193 and 157 nanometers, ‘soft’ x-ray (so-called extreme ultraviolet (EUV), 13.5 nm) and x-ray radiation, and beams of ions or electrons. Patterns are defined by irradiation through a patterned mask (in the case of optical, EUV, x-ray, and projection electron beam lithography) or via a direct write process in the case of electron or ion beam lithography. The incident radiation induces a chemical change in the photoresist film which causes a physical (e.g., molecular weight or thermal stability) or chemical (solubility) property of the exposed material to differ from that of material in the unexposed regions. Subsequent development processes can selectively remove the material in either the exposed or unexposed region. Typically, this involves rinsing the exposed silicon wafer with a developer such as aqueous tetramethylammonium hydroxide.
Photoresists are generally formulated to contain a matrix polymer, a radiation sensitive compound/functionality, and performance modifiers (e.g., dissolution inhibitors and bases quenchers) in a solvent suitable for spin-casting. While early photoresists relied on direct interaction of incident radiation with the radiation sensitive compound/functionality, the low quantum efficiency of this approach is unsuitable for high resolution imaging in which high sensitivity to low doses of radiation is required. Subsequently, “chemically-amplified” resists have been developed in which the incident radiation interacts with a radiation sensitive compound/functionality to produce a species capable of performing a catalytic reaction on a large number of functional groups to induce a large property change from a low exposure dose. Typically, chemically-amplified resists are designed with a compound, referred to as a photoacid generator (PAG), which produces a strong acid when exposed to radiation of the appropriate wavelength. This strong acid catalyzes chemical reactions such as the deprotection of acid-labile protecting groups (typically positive tone photoresists) or the polymerization of acid-sensitive groups such as epoxides or the reaction of polymer-bound functionalities with crosslinking agents (negative-tone photoresists). In this manner the quantum efficiency of the overall process can approach or even exceed a value of one. The particular chemical structures of the functional groups attached to the matrix polymer is particularly important since it, typically, defines the tone (positive or negative) and imaging performance of the photoresist.
In practice, many properties of the photoresist and its components determine its imaging performance. The non-radiation sensitive components of the formulation must be relatively transparent (particularly with optical lithography) to avoid non-productive attenuation of the incident radiation. In optical lithography, the ultimate achievable resolution is a function of the wavelength of the incident radiation according to the Rayleigh equation:R=k1λ/NA  (1)where λ is the wavelength of the incident radiation, NA is the numerical aperture of the lens system, and k1 is a process-dependent factor typically between 0.5 and 0.25. In order to achieve smaller feature sizes, industry has traditionally moved to shorter wavelengths of light, often requiring redesign of photoresists to accommodate the new radiation wavelength. For example, photoresist polymers based on 4-hydroxystyrene and its copolymers are widely used with 248 nm radiation due to their high resistance to etch processes and high transparency; however, these aromatic polymers are not useful for imaging with 193 nm radiation due to their heavy absorbance. Subsequently, new photoresists based on acrylate, methacrylate, or cyclic olefin polymers were developed which are transparent at 193 nm. Carbon-rich, alicyclic groups have been particularly featured due to their good transparency and high etch resistance.
In order to achieve even smaller features using the same incident radiation source, immersion lithography has been developed in which a medium with a refractive index greater than air (or vacuum) is placed between the final lens element and the photoresist. Most commonly, this involves the use of water in 193 nm immersion lithography; however, higher index aqueous and organic fluids have been demonstrated for 193 nm and 157 nm immersion lithography. The chemistry of the photoresist must be tailored such that the photoresist does not dissolve or swell in the immersion medium. Sometimes a sacrificial protective topcoat film is deposited on top of the photoresist to prevent leaching of photoresist components (e.g., PAG, quencher, matrix resin) into the immersion medium. The topcoat also controls the surface energy of the film stack, controlling the contact angle of the immersion fluid in contact with the surface. High fluid contact angles have been shown to facilitate rapid scanning of the immersion lens across the wafer surface without leaving residual droplets of fluid behind, which can lead to defects. The chemistry and the topcoat material are matched to afford good compatibility and imaging performance.
Since the dimensions of image features and characteristics such as line edge roughness (or line width variation) are of the same order as the radius of gyration of typical polymeric photoresist matrix resins, new architectural variants of the polymeric matrix resins have been developed including branched and hyper-branched polymers and oligomers. In the extreme case, functionalized-monomeric compounds (so-called “molecular resists” or “molecular glasses”) based on well-defined compounds such as cyclodextrins, polyhedral oligomeric silsesquioxanes, or adamantane have been functionalized and implemented in photoresist compositions.
Unfortunately, changes in the chemistry of photoresist materials affect not only the transparency of the photoresist but its dissolution behavior during development. Poly(4-hydroxystyrene)-based 248 nm photoresists dissolve uniformly in industry standard 0.262 N tetramethylammonium hydroxide developer with no swelling at the onset of dissolution. Further modification of the dissolution behavior is possible through the use of protecting/crosslinking groups and dissolution inhibitors. However, typical acrylate, methacrylate, and cyclic olefin photoresists used with 193 nm radiation swell during the initial stages of development, affording non-uniform dissolution. Photoresists based on fluoroalcohols have been developed for 157 nm and 193 nm imaging. The inductive stabilization of the conjugate base of these alcohols by the heavily electron-withdrawing groups can result in a pKa similar to that of phenol and can render these functional groups soluble in aqueous base. Fluoroalcohol based resists have been shown to offer linear dissolution behavior with little swelling.
Another possible approach to improve the ultimate resolution of photoresists is to bake them at low temperatures after exposure. In this manner, the diffusion of the photoacid can be limited and can result in lower image blur. This approach necessitates the use of a protecting group with a low activation energy for acid-catalyzed deprotection. It has also been shown in the literature that the rate of acid-catalyzed reactions in the photoresist is heavily affected by the polarity of the medium (i.e., higher polarity media facilitates more rapid reactions at lower temperatures and lower polarity media requires higher temperatures to achieve similar reaction rates). Unfortunately, many of the common acid-labile protecting groups are simple hydrocarbons which reduce the overall polarity of the photoresist film and raise the required post-exposure bake temperatures and increase image blur.
Photoresists based on fluoroalcohols have been developed for 157 nm and 193 nm imaging have been described in the patent literature. A class of tertiary acrylate esters containing fluoroalcohol groups has been reported in JP 2005132827. However, the presence of heavily electron-withdrawing fluorinated groups adjacent to the ester in these compounds will hinder the formation of the intermediate carbocation during deprotection to significantly limit the cleavage of the protecting group under acid catalysis and substantially interfere with the performance of the resist. Several types of tertiary acrylate esters and acetals containing fluoroalcohol groups have been described in JP 2005099276, US 2005026074, JP 2005004159, JP 2004069981, and JP 2004069921. These monomers generally have multiple fluoroalcohol groups which will impart very high solubility (and dark loss) to the resist. Acetal-based protecting groups require water to be present for the deprotection reaction and therefore are disfavored in the industry. Several other monomers require long, inefficient syntheses employing oxidation/reduction sequences and/or rely on non-commercially available starting materials. There is a need for lithographic methods using improved photoresists which can be synthesized on a commercial scale with commercially available materials to provide enhanced lithographic performance.